RFC 6512

Using Multipoint LDP When the Backbone Has No Route to the Root

Internet Engineering Task Force (IETF) IJ. Wijnands
Request for Comments: 6512 E. Rosen
Category: Standards Track Cisco Systems
ISSN: 2070-1721 M. Napierala
AT&T
N. Leymann
Deutsche Telekom
February 2012 Using Multipoint LDP When the Backbone Has No Route to the Root
Abstract
The control protocol used for constructing Point-to-Multipoint and
Multipoint-to-Multipoint Label Switched Paths ("MP LSPs") contains a
field that identifies the address of a "root node". Intermediate
nodes are expected to be able to look up that address in their
routing tables. However, this is not possible if the route to the
root node is a BGP route and the intermediate nodes are part of a
BGP-free core. This document specifies procedures that enable an MP
LSP to be constructed through a BGP-free core. In these procedures,
the root node address is temporarily replaced by an address that is
known to the intermediate nodes and is on the path to the true root
node.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6512.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of

1. Introduction
The document [mLDP] extends LDP [LDP] to support multipoint Label
Switched Paths. These extensions are known as "Multipoint LDP", or
more simply, as "mLDP". [mLDP] defines several LDP Forwarding
Equivalence Class (FEC) element encodings: "Point-to-Multipoint"
(P2MP), "Multipoint-to-Multipoint (MP2MP) Upstream", and "MP2MP
Downstream".
The encoding for these three FEC elements, as defined in [mLDP], is
shown in Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Address Family | Address Length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Root Node Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
~ ~
| Opaque Value |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: mLDP FEC Element Encoding
Note that a P2MP or MP2MP Label Switched Path ("MP LSP") is
identified by the combination of a "root node" and a variable length
"opaque value". The root node also plays a special role in the mLDP
procedures: mLDP messages that are "about" a particular MP LSP are
forwarded to the LDP adjacency that is the next hop on the route to
the root node.
Sometimes, it is desirable for an MP LSP to pass through a part of
the network in which there is no route to the root node. For
instance, consider the topology of Figure 2.
CE1----PE1---P1---- ...----P2 ----PE2----CE2----R
Figure 2
In Figure 2, CE1 and CE2 are "Customer Edge routers", R is a customer
router at the same VPN site as CE2, and PE1 and PE2 are "Provider
Edge routers". Suppose that PE1 has a BGP-learned route for R, with

PE2 as the BGP next hop. Suppose also that the provider's interior
routers (such as P1 and P2) do not have any BGP-learned routes and,
in particular, do not have any routes to R.
In such an environment, unicast data packets from CE1 addressed to R
would get encapsulated by PE1, tunneled to PE2, decapsulated by PE2,
and forwarded to CE2.
Suppose now that CE1 is trying to set up an MP LSP whose root is R,
and the intention is that the provider's network will participate in
the construction of the LSP. Then, the mLDP messages identifying the
LSP must be passed from CE1 to PE1, from PE1 to P1, ..., from P2 to
PE2, from PE2 to CE2, and from CE2 to R.
To begin the process, CE1 creates an MP FEC element with the address
of R as the root node address and passes that FEC element via mLDP to
PE1. However, PE1 cannot use this same FEC element to identify the
LSP in the LDP messages it sends to P1, because P1 does not have a
route to R.
However, PE1 does know that PE2 is the BGP next hop on the path to R.
What is needed is a method whereby:
- PE1 can tell P1 to set up an LSP as if the root node were PE2,
- PE2 can determine that the LSP in question is really rooted at R,
not at PE2 itself, and
- PE2 can determine the original FEC element that CE1 passed to PE1,
so that PE2 can pass it on to CE2.
This document defines the procedures that allow CE1 to create an LSP
rooted at R. These procedures require PE1 to modify the MP FEC
element before sending an mLDP message to P1. The modified FEC
element has PE2 as the root and the original FEC element as the
opaque value. This requires a new type of opaque value. Since the
opaque value contains a FEC element, we call this a "Recursive Opaque
Value". When PE2 sends an mLDP message to CE2, it replaces the FEC
element with the opaque value, thus undoing the recursion. Details
are in Section 2.
Section 3 defines the "VPN-Recursive Opaque Value". Whereas the
"Recursive Opaque Value" carries the original FEC, the "VPN-Recursive
Opaque Value" carries the original FEC plus a Route Distinguisher
(RD). This is applicable when MP LSPs are being used to carry the
multicast traffic of a VPN [MVPN]. Details are in Section 3.

The value field of the VPN-Recursive Opaque Value consists of an
8-octet Route Distinguisher (RD), followed by a P2MP or MP2MP FEC
element, encoded exactly as specified in [mLDP], with a type field, a
length field, and value field of is own. The length of the
VPN-Recursive Opaque Value thus includes the 8 octets of RD plus the
lengths of the type, length, and values fields of the contained FEC
element.
3.2. Procedures
3.2.1. Non-Segmented Inter-AS P-Tunnels
Consider the inter-AS (Autonomous System) VPN scenario depicted in
Figure 5.
PE1 --- P1 ---- ASBR1 ... ASBR2 ---- P2 ---- PE2
Figure 5
Suppose this is an "option B" VPN interconnect ([VPN], Section 10).
This means that the Autonomous System Border Router (ASBR) in the
first Autonomous System (i.e., ASBR1) does not have a route to PE
routers in other ASes (such as PE2). Suppose also that the Multicast
VPN (MVPN) policy is to instantiate Provider Multicast Service
Interfaces (PMSIs) [MVPN] using mLDP and that "non-segmented inter-AS
P-tunnels" [MVPN] are being used.
In this scenario, PE1 may need to join a P2MP or MP2MP LSP whose root
is PE2. P1 has no route to PE2, and all PE1 knows about the route to
PE2 is that ASBR1 is the BGP next hop. Since P1 has no root to PE2,
PE1 needs to originate an mLDP message with a FEC element that
identifies ASBR1 as the root. This FEC element must contain enough
information to enable ASBR1 to find the next hop towards PE2 even
though ASBR1 does not have a route to PE2.
Although ASBR1 does not have a route to PE2, it does have a BGP
Intra-AS Inclusive PMSI (I-PMSI) auto-discovery (A-D) route [MVPN]
whose Network Layer Reachability Information (NLRI) contains PE2's IP
address together with a particular RD. PE1 also has this Inter-AS
I-PMSI A-D route. The LSP needs to be set up along the path
established by the Intra-AS I-PMSI A-D routes. Therefore, one must
use a Recursive FEC element that contains the RD as well as the
address of PE2. The "VPN-Recursive FEC Element" defined herein is
used for this purpose.

This enables us to provide the same functionality for mLDP P-tunnels
that is provided for PIM P-tunnels in Section 8.1.3.2 of [MVPN]
through the use of the MVPN Join Attribute.
At PE1 in Figure 4, the LSP to be created is associated with a
particular VPN Routing and Forwarding Table (VRF). PE1 looks up in
that VRF the Intra-AS I-PMSI A-D route originated by PE2. It finds
that the BGP next hop of that route is ASBR1. So, it creates a P2MP
or MP2MP FEC element whose root is ASBR1 and whose opaque value is a
VPN-Recursive FEC element. The VPN-Recursive FEC element itself
consists of a root, an RD, and an opaque value, set as follows:
- The root is PE2.
- The RD is the RD from the NLRI of the Intra-AS A-D route
originated by PE2.
- The opaque value is chosen (by some method outside the scope of
this document) so as to be unique in the context of PE2. (For
example, it may have been specified in a PMSI Tunnel Attribute
originated by PE2.) We will refer to this opaque value as "Q".
The resulting FEC element can be informally represented as
<root=ASBR1, opaque_value=<root=PE2, RD, opaque_value=Q>>.
PE1 can now begin setting up the LSP by using this FEC element in an
LDP Label Mapping message sent towards ASBR1.
When ASBR1 receives, over a non-VRF interface, an mLDP Label Mapping
message containing this FEC element, it sees that it is the root and
that the opaque value is a VPN-Recursive Opaque Value. It parses the
VPN-Recursive Opaque value and extracts the root value, PE2.
If ASBR1 has a route to PE2, it continues setting up the LSP by using
the following FEC element:
<root=PE2, opaque_value=Q>
However, if ASBR1 does not have a route to PE2, it looks for an
Intra-AS I-PMSI A-D route whose NLRI contains PE2's address along
with the specified RD value. Say the BGP next hop of that route is
ASBR2. Then ASBR1 continues setting up the LSP by using the
following FEC element:
<root=ASBR2, opaque_value=<root=PE2, RD, opaque_value=Q>>.

Note that in this case, the root has changed from ASBR1 to ASBR2, but
the opaque value is the unchanged VPN-Recursive FEC element.
3.2.2. Limited Carrier's Carrier Function
Another possible use of the VPN-Recursive FEC is to provide a limited
version of "Carrier's Carrier Service". Referring again to the
topology of Figure 2, suppose that PE1/PE2 are offering "Carrier's
Carrier VPN Service" [VPN] to CE1/CE2. CE1 sends PE1 an MP FEC
element whose root node is R and whose opaque value is Q. We will
refer to this FEC element as "CE1-FEC". However, PE1's route to R
will be in a VRF. Therefore, the FEC element created by PE1 must
contain some identifier that PE2 can use to find the proper VRF in
which to look up the address of R.
When PE1 looks up the address of R in a VRF, it will find a route in
the VPN-IP address family. The next hop will be PE2, but there will
also be a Route Distinguisher (RD) as part of that NLRI of the
matching route. In this case, the new FEC element created by PE1 has
the address of PE2 as the root node address and has a VPN-Recursive
Opaque Value. The value field of the VPN-Recursive Opaque Value
consists of the 8-octet RD followed by CE1-FEC.
As far as the interior routers are concerned, they are being
requested to build an MP LSP whose root node is PE2. They MUST NOT
interpret the opaque value at all.
When an mLDP Label Mapping message containing PE2-FEC arrives at PE2
over a VRF interface, PE2 notes that it is the identified root node
and that the opaque value is a VPN-Recursive Opaque Value.
Therefore, it MUST replace PE2-FEC with the contents of the
VPN-Recursive Opaque Value (i.e., with CE1-FEC) before doing any
further processing. It uses the VRF to look up the path to R. This
will result in CE1-FEC being sent on to CE2, and presumably further
from CE2 to R.
In this scenario, the RD in the VPN-Recursive Opaque Value also
ensures uniqueness of the FEC element within the inner carrier's
network.
This way of providing Carrier's Carrier service has limited
applicability, as it only works under the following conditions:
- Both the inner carrier and the outer carrier are using non-
segmented mLDP P-tunnels.

- The inner carrier is not aggregating the P-tunnels of the outer
carrier but is content to carry each such P-tunnel in a single
P-tunnel of its own.
The Carrier's Carrier scenario can be distinguished from the inter-AS
scenario by the fact that in the former, the mLDP messages are being
exchanged on VRF interfaces.
4. IANA Considerations
[mLDP] defines a registry for "The LDP MP Opaque Value Element Basic
Type". Two new code points have been assigned in this registry:
- Recursive Opaque Value: Type 7
An opaque value of this type is itself a TLV that encodes an mLDP
FEC type, as defined in [mLDP].
- VPN-Recursive Opaque Value: Type 8
An opaque value of this type consists of an 8-octet Route
Distinguisher as defined in [VPN], followed by a TLV that encodes
an mLDP FEC type, as defined in [mLDP].
5. Security Considerations
The security considerations of [LDP] and [mLDP] apply.
Unauthorized modification of the FEC elements defined in this
document can disrupt the creation of the multipoint LSPs or can cause
the multipoint LSPs to pass through parts of the network where they
are not supposed to go. This could potentially be used as part of an
attack to illegitimately insert or intercept multicast traffic.
However, since the FEC elements defined in this document are not
inherently more vulnerable to this form of attack than are the
previously defined FEC elements, this document does not add new
security vulnerabilities.
A description of general security issues for MPLS can be found in
[RFC5920].
6. Acknowledgments
The authors wish to thank Toerless Eckert for his contribution to
this work.